The present invention is directed to a method and apparatus for non-destructive testing and evaluation of materials, and more particularly to a method and apparatus for identifying and registering defects in a sample by superimposing a defect image over a live image to locate subsurface defects in the sample.
Various methods of non-destructive testing and evaluation (NDT/E) of parts have been developed to detect subsurface defects in a part sample and to measure the depth of subsurface defects. These methods include step thermography, pulse thermography, and other thermographic techniques. All of these techniques involve deliberately changing the temperature of the sample, and observing the temperature change of the sample via an infrared (IR) camera as it returns to equilibrium temperature. Anomalous temperature changes that appear in the infrared camera image indicate subsurface defects in the sample; subsurface defects tend to impede the normal heat flow in the sample and will appear as anomalies in the image. Further, because the infrared image showing the defect is transient and may last for only a fraction of a second, the image must be captured (usually with a digital computer) and then verified against the actual sample to locate the exact position of the defect.
The actual verification process, usually through a complementary NDT/E process, can be relatively difficult because the infrared defect image of the sample may bear little resemblance to the actual sample. For example, many subsurface defects appear only in the infrared image; to the naked eye, the sample containing the defects often appears perfectly uniform. As a result, a user must attempt to match the infrared image of the subsurface defect with the actual, unblemished sample surface to pinpoint the location of the defect. This is further complicated by the fact that the infrared camera lens often distorts the image, causing straight lines at the periphery of the lens""s field of view to appear curved in the image. To locate and mark the positions of subsurface defects with some precision, prior art methods include using regularly spaced registration markers on the sample, calculating complex anamorphic mapping algorithms, or printing a full-size defect image and physically matching or overlaying the full-size image onto the actual sample. Because the sample may not have any distinguishing marks that appear in the defect image, precise registration of the image and the sample""s surface can be difficult. In addition, these methods are time-consuming and are not particularly convenient, and at best they can only approximate the subsurface defect location due to the image distortion from the infrared camera lens. Further, measuring the depth of subsurface defects often requires some prior knowledge of the sample""s dimensions or properties, such as the thickness of the sample, the depth of a known defect, the material""s thermal diffusivity, etc. This information is often not available in practice, making precise depth measurements difficult with known techniques.
Thus, there currently is a need for a NDT/E technique that allows accurate annotation, marking, and thickness measurements of specific locations on a sample, without the problems caused by differences between the image and the actual sample due to image distortion. There is also a need for a NDT/E technique that can conduct depth measurement without requiring prior knowledge of any of the sample""s characteristics. If the depth and onset time (relative to the heating event) of a single defect are known, it is then possible to calculate the thermal diffusivity of the material and to use it to determine the depths of other defects from their offset time, according to the well-known relation of t=d2/D, where t is the onset time, d is the depth of the defect, and D is the thermal diffusivity of the sample.
Accordingly, the present invention is a method and apparatus for conducting NDT/E that simplifies the correspondence between image information obtained during NDT/E and the actual sample. More specifically, the invention involves obtaining a defect image and a live image of the same sample and then superimposing one image on the other. One embodiment of the invention is directed to linking the information regarding surface defects obtained during infrared NDT/E to the actual part being inspected. The invention includes generating a defect image of the sample via infrared imaging or some other means. The defect image may have markers or other indicia locating where subsurface defects are in the sample. The defect image is then superimposed onto a live image of the sample. A user then views the live image of the sample, rather than the sample itself, while transferring the marks from the defect image to the sample. Because both the defect image and the live image are distorted by the infrared camera lens and therefore have a one-to-one correspondence, the distorted image is used as the frame of reference for locating subsurface defects and marking the sample. This ensures that the marks in the defect image are transferred precisely from the defect image onto the sample and also eliminates the need to map the distorted image to the sample in a separate step.